Tunable oscillatory circuit for ultra-short waves



May 20, 19 41 K. SCHUSSLER TUNABLE OSCILLATORY CIRCUIT FOR ULTRA-SHORT WAVES Filed Nov. 1a, 1939 Eye IIIIIIII/IIIIIIIIIIIIIIIII/IIIIIIIIIIIIIIII/IIIIIIII/ INVENTOB KARL SCHUSSLE ATTORNEY Patented May 20, 1941 TUNABLE OSCILLATOR]? CIRGUIT FOR ULT-RA- SHORT WAVES Karl Schiissler, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application November 18, 1939, Serial No. 305,089 In Germany October 19, 1938 Claims.

The present invention is concerned with a tunable circuit for ultra-short waves which consists of a double-wire line being changeable in its length and preferably short-circuited unilaterally. Fundamentally speaking, it is known in the art to substitute for the concentrated or lumped inductances customarily employed for longer waves distributed inductances and capacities of a double-wire or coaxial line whenever short waves are dealt with. Thus, oscillatory circuits for ultra-short waves in past practice consisted of a double-wire line which, in the majority of cases, was short-circuited at one end and the length of which was varied with a view to tuning the same. The highest resistance of the tuning circuit, that is, tuning to parallel resonance, arises in the presence of a length of the short-circuited line equal to M4. If the wavelength is relatively high also the absolute length of this oscillatory circuit will be great and thus also the percentage precision of sharpness of the tuning, as the short-cirouiting bridge of the line may be shifted very small amounts compared to the aggregate length of the line. In the case of shorter lines for which the absolute length of the oscillation circuit length becomes gradually smaller, however, even the most minute shifts of the short-circuiting bridge or slider will result in high percentage detuning of the oscillatory circuit. Where ultra-short Waves are dealt with, serious constructional difiiculties are encountered for these reasons in the attempt of rendering the oscillation circuits tunable; in fact, these diiiiculties prove insurmountable in a great many instances. Now, the present invention has the aim and object to obviate these difiiculties.

According to the invention, the double-wire line serving for the oscillation circuit comprises two or more parts of dissimilar characteristic impedance rather than one length of constant characteristic impedance, the tuning being efiected in this oscillation circuit by a change or shift in position of the part possessing the maximum characteristic impedance.

As a general rule, that is to say, in the case of lines with a homogeneous characteristic impedonce, the tuning is independent of the characcharacteristic impedance contributes but little, comparatively speaking, to the aggregate electrical length. Thus, on changing the tuning by change'of length of the part possessing .the higher characteristic impedance, it will be seen that a substantially greater sharpness of setting and tuning is attainable.

Assuming that in a line composed of two parts the portion of higher characteristic impedance is of small length compared with the part possessing the lower characteristic impedance, it follows that in the presence of resonance of the entire system, that is to'say, for an aggregate electrical length of M4, there holds good for the length 12 of the part possessing the higher characteristic impedance the following especially simple equation:

ia. Z1 21! 27F-l1 where Z1 the characteristic impedance of the low-impedance part, Z2 the characteristic impedance of the other portion of the line, A the wavelength, and 11 the length of the part possessing characteristic impedance Z1. With close approximation, length Z2 is proportional to the characteristic impedance Z2. For the same frequency change the variation of the length 12 has risen at the ratio 22/21. In other words, the change in length of the high-impedance part will become less noticeable at the ratio Z2/Z1 in the tuning than in a length variation of the other line part. However, this means an increase in the sharpness of tuning. If, in the case of a coaxial line, the ratio Z2/Z1 is chosen, for instance, at 3, and this is practically readily accomplishable, this means that the sharpness of the tuning will be three times higher than in the case where the line is homogeneous. Indeed, practical tests have fully confirmed this theory and consideration.

The invention shall now be described in more detail by reference to Figs. 1 and 2 of the appended drawing in which Fig. 1 is a diagram of the invention and Fig. 2 is a longitudinal section of a device of the invention. Fig. 1 shows the basic principle of an oscillatory circuit consisting of coaxial conductors whose inner conductor consists of two parts J1 and J2 having dissimilar diameters. The line is unilaterally short-circuited, so that it forms what has been called a pot circuit, which is known to those skilled in the art as a concentric line section closed at one end thereby forming a pot-like configuration. At the end opposite the short-circuited, the outer conductor, if desired, and as illustrated, may be provided with a closing plate. Coupling relationship between this oscillatory circuit and some other circuit is established, for instance, by the loop N which is introduced into the interior of the pot circuit in a way so as to insulate it therefrom. A change in the tuning, for example, is accomplishable by shifting in of the part Ji. In such a scheme, lengths l1 and 12 of the two parts obey the above-mentioned formula as long as the premises mentioned in connection with the said formula are fulfilled. If the presupposition that 12 is small in contrast to Z1 is no longer fulfilled, the theoretical situation assumes a more complex character, though the basic idea of the invention is applicable still in the same way. A circuit practically designed as shown in Fig. 1 comprised a part J1 of 50 mm. length and a part J2-0f 8 mm. length. The length of the wave to which this circuit was tuned amounted to roughly six centimeters. The circumstance which is striking for this; wave-length is that the geometrical length of the, oscillation-circuit line is not over M4 in line with whathad to be expected on the basis of theoretical considerations, and this due to the fact that the geometrical length is reduced because of the capacitances arising atthe junction point fromJz to..J1 aswell as at the end of J2, with, the result that, the aggregate-lengtnoi thesystem with fairly close accuracy corresponds to a wave-lengthof M4.

The invention is particularly suitedfor wavemeter circuits where the demands regarding accuracy are particularly severe. A wave-meter of this kindis shown in Fig. 2 which corresponds to the arrangement Fig. 1, though showing more clearly the mechanism causative of the change in length of part J2. This part J2, as will be seen, is arranged to be shifted longitudinally by the aid of a spindle or worm Sp which is journaled in the screw-thread G 'of part J1. D denotes a spiral spring designed to avoid slight inaccuraciesiin adjustment. As to the rest, it will be superfluous to give a detailed description of 'Fig. for this embodiment of the invention will, be self-explanatory.

The invention is not in fact, it may similarly-beadopted in connection with double-wire lines.

What is claimed is:

1. A tunable oscillatory circuit comprising a confined to coaxial'lines,

section of a concentric line having an outer conductor and an inner conductor, said inner conductor being composed of two parts of appreciably difierent diameters, a connection between the inner conductor and the outer conductor at one end, the free end of said inner conductor being of smaller diameter and'adjustable with respect to the fixed end.

2. A tunable oscillatory circuit comprising a section of a concentric line having an outer conductor and an inner conductor, said inner conductor being composed of two parts each of a different diameter, a connection between the inner conductor and the outer conductor at one end, a threaded spindle within said inner conductor, the free end of said inner conductor being of smaller diameter than the end connected to. said outer conductor and being adjustable by said threaded spindle with respect to the connected end.

3. A tuned circuit comprising a concentric line resonator having an inner and an outer conductor, said inner conductor having two sections of appreciably different diameters, means coupling the larger diameter section to said outer conductor to provide a path of low impedance therebetween for energy of the operating frequency, and means for adjusting the efiective length of the smaller diameter section to change the overall length of said inner conductor.

4. A tuned circuit comprising a concentric line resonator having an inner and an outer conductor, said inner conductor. having two overlapping sections of appreciably difierent diameters, means coupling the larger diameter section to said outer conductor to provide a path of low impedance therebetween for energy of the operating frequency,'and means located externally of said resonator and linked to the smaller diameter section for adjusting the length of said smaller diameter section outside the larger diameter section.

5. A tuned circuit comprising a pair of parallel conductors short-circuited at one end for KARL SCHiJSSLER' 

